Weird Javascript and AJAX errors

I recently implemented some JavaScript logging on one of the web sites that I work on. Something like this: Using XMLHttpRequest to log JavaScript errors. It is working well and helping me uncover errors in my code, but many of the remaining errors that I see make little or no sense. Here is a list of some of the oddities that I have seen, that have been unable to reproduce in my development environment since that are so rare and sporadic:

1. There are times that certain JavaScript functions and variables cannot be found. Many of these are defined in external JavaScript files. In the case of Firefox I see errors when an external file fails to load for some reason. Internet Explorer give no such indication, but I have to assume that the same thing is happening. The Firefox errors do not give any details as to why the file failed to download.
   
2. Sometimes the server side logging gets blank errors. So somehow my logging page gets hit with no data, this shouldn't be happening.
3. The most frequent AJAX error is when making an AJAX call, the data returned is incomplete. I know that the readyState property is set to 4 and the status is 200, but looking at the actual length of the data (from the Content-length in the header) and comparing it to the length of the data in responseText, some data is missing. Sometimes it is almost the right size but many times it is only a fraction of the expected size. This is even after taking into account the fact that the data is UTF-8 encoded. The data size can be anywhere from 20 to 30KB, so I have wondered if the amount of data may be a contributing factor.
   
4. The other AJAX error is non-standard status codes. With FireFox I see responses of 0, and Internet Explorer I see the infamous 12000 error codes like 12019 12029 12030 and others.

So far here is what I have done to reasearch and or resolve these issues:

1. I haven't found anything definitive to help when files fail to load, but I am going to enable gzip compression for javascript files on IIS 7 to see if it might be to due to slow connections timing out. Hopefully the smaller file size will help these requests succeed more often, but this is not a complete solution. I expect to continue to see this problem.
   
2. This one has me stumped. The JavaScript logging code should always be passing an error message, with the error data, even the JavaScript error handling has no content. I have no idea why these would come back blank. Maybe the request is timing out, or I have an issue with the server side logging code not waiting until all of the data is ready.
   
3. I haven't had much luck with this one either. So far I have added some retry code, so if I get a failure, I just try again. This appears to work about 75% of the time, but I currently limit it to 1 retry, so I still see some failures. This also seems like a less than ideal solution, but maybe it is the best I can do.
   
4. Same as number 3, I just try the request again, and is succeeds about 75% of the time. The requests are over HTTPS and some information I've found indicate that this might be a problem with Internet Explorer trying to reuse connections and failing, but I have not tried adding the Connection: Close header yet.
   

These all seem very odd to me. Most of them occur on both Firefox and Internet Explorer 6, 7, and 8, but very infrequently in almost all cases, but each case still represents someone that will probably have a poor customer experience on the web site. I have to assume that errors like this are pretty common on other sites, but how many people have JavaScript logging set up that would even notice them.

Death to Internet Explorer 6

I personally think that Internet Explorer 6 should be outlawed. Web development can be difficult enough to make things look good and work right, that throwing Internet Explorer 6.0 into the mix just makes things that much harder. Even when I'm doing ASP .NET development, you would think that everything would work well with IE 6, but that is not the case.

I spend most of my time using Firefox to test my sites, then do some quick checks in either IE 7 or IE 8, depending on what is installed on the computer, and in most cases things look and work pretty well. Sometimes there may be a few tweaks necessary to get things just right. After that I have to spin up a virtual machine, or find an old computer with IE 6 on it. And that is where the fun begins.

Web page layouts never quite look right, IE 6 never really seems to do what you've told it to. It selectively ignores CSS and re-sizes things how it wants. The internet is abound with IE 6 CSS hacks. Functionality seems to have just as many problems. Basic JavaScript is hit or miss, it might work just fine, or it might decide to be your worst enemy. Anything more complex like AJAX is almost a lost cause. You might as well develop and maintain two separate websites: one for real web browsers and another for IE 6.

Maybe I'm being a little hard on the browser, but it really is a web developers worst nightmare. If there were only a few computers out there that still had IE 6, that would be one thing. But there is still a large portion of computers that run IE 6 as their primary browser. I defintely favor Firefox, but I don't mind if people want to use IE 7 or IE 8, just not IE 6. We should all wish it a fond farewell, and retire the old chap already.